The U.S. life span has been steadily increasing -- although the rate of increase has slowed during the last 30 years. How long can we live? We may not get to find out, suggest S. Jay Olshansky, PhD, of the University of Illinois in Chicago; David S. Ludwig, MD, PhD, of Children's Hospital in Boston; and colleagues.

The researchers calculate that in the first half of this century, U.S. life expectancy will level off or get shorter. Yes, medical advances will reduce death rates from various diseases. But they predict that onrushing disaster will overshadow all these advances.

"The childhood obesity epidemic is an impending catastrophe," Ludwig tells WebMD. "Nothing like this has happened before. It will overwhelm any other changes we will make to affect longevity."

The researchers' special report appears in the March 17 issue of The New England Journal of Medicine.

Child Obesity Threatens U.S. Life Span

Ludwig and colleagues note that two-thirds of American adults are obese or overweight. That dramatically increases our chances of type 2 diabetes, heart disease, cancer, kidney disease, and other life-threatening conditions.

But this is child's play compared with the effects of childhood obesity on life span. America's childhood obesity problem is growing faster and faster.

"Obesity rates are increasing fastest among children, and they will carry obesity-related health risks throughout their lives," Ludwig says. "An adult who gains a pound or 2 a year through middle age will be at increased risk. But that is much less dire than the overweight 4- to 6-year-old who gets diabetes at age 14 or 16 and has a heart attack before age 30."

Ludwig -- director of the obesity program at Children's Hospital, Boston -- says the childhood obesity epidemic has three phases. The first came in the last decade, when child obesity became common but the public health effects weren't yet felt. Phase two is right now, as we begin to see serious complications such as type 2 diabetes in very young people. Phase three, Ludwig predicts, is coming soon.

"But we still have a little time before these children become young adults with diabetes and start to have heart attacks, stroke, kidney failure, and increased mortality," he says. "It is a massive tsunami headed for the United States. One can know it is coming. But if we wait until we see the ocean level rising over the shore, it will be too late to take action."

In an editorial accompanying the study, University of Pennsylvania demography expert Samuel H. Preston, PhD, agrees that the Olshansky/Ludwig team's calculations are accurate. But Preston disagrees with their conclusion.

"I am optimistic," Preston tells WebMD. "The U.S. lifespan has shown remarkably steady progress for a century in the U.S. And we have demonstrated that when we get to a point where lifestyle factors seriously affect national mortality, we are able to move in the proper direction."

As an example, Preston points to the epidemic of cigarette smoking that set back U.S. life expectancy in the 1950s and 1960s. A huge public effort cut smoking rates -- and Americans' life spans renewed their upward march.

Ludwig hopes Preston is right. But he says the U.S. response to childhood obesity falls far short of the U.S. antismoking effort.

"We continue to condone a multibillion-dollar campaign by the food industry to get children to eat the most unhealthy foods imaginable," Ludwig says. "We don't adequately fund schools, so principals have to turn to soda machines in the hallways and fast-food contracts in the cafeterias to close budget gaps. At the same time, lack of funds forces them to close afterschool activities and physical education."

The childhood obesity expert calls for a ban on food ads aimed at young children. He calls for stronger federal funding of schools -- especially for mandatory physical education programs. And he calls for federal laws to force insurance companies to pay for the treatment of childhood obesity.

But Ludwig doesn't let families off the hook.

"Parents can turn off the TV and have a family meal once a day together -- that is at least one opportunity to give children healthy food and model a healthy lifestyle," he says.

Preston, too, asks Americans to act.

"I think WebMD readers already know what the problem is: You are hurting yourselves and your children via poor diet and inadequate exercise levels," he says. "The problem already is serious enough to impact national mortality figures. So look to yourselves."

Will we take action before it's too late? Preston thinks we will.

"On a long-term basis, I do not foresee a cessation in the rate of improvement in mortality," he says. "Our institutions are well adapted to advancing health and longevity. They have shown an amazing ability to do so over a century. I do not see why that period should come to an end."